High speed switching solid state relay circuit

ABSTRACT

A method for high speed switching comprises receiving voltage inputs at a bridge rectifier, receiving a logic input signal at an optical isolator and generating an output signal from the optical isolator based on the logic input signal, and driving a gate of a field effect transistor (FET) via the output signal of the optical isolator, wherein a source of the FET is connected to a negative output of the bridge rectifier and a drain of the FET is connected to a positive output of the bridge rectifier through a load. The method further includes limiting current flowing to the gate of the FET through first and second resistors and first and second diodes connecting the voltage inputs to the gate of the FET and limiting voltage to the gate of the FET below a maximum voltage rating of the FET by a Zener diode connected to the gate of the FET.

FIELD

The disclosure relates generally to switching circuits and in particular to high speed switching solid state relay circuits.

DESCRIPTION OF THE RELATED ART

Advances in solid-state switching and relay technology have made possible the replacement of many electro-mechanical switching and relay assemblies. Solid-state devices provide the power control systems in which they are incorporated with long life, quiet operation and other associated advantages.

The solid state relay is a non-contact electronic switch-off for the four-terminal active device, with two input and two output terminals. With a DC pulse signal coupled to the input terminals, the output can be in an off state or conductive state, that is, when a voltage is applied between the input terminals, the solid state relay is turned on, and when voltage between the input terminals is withdrawn the solid state relay is turned off. Since the solid state relays do not have contact switch elements, compared with the electro-mechanical relays, they have advantages such as better reliability, longer life, freedom from outside interference, anti jamming capability, compatibility with logic circuits, high speed switching etc.

However, existing solid state relays have some disadvantages. They require several types of protection, including high voltage and high current protection. Solid-state relays cannot be maintained and have to be replaced if damaged. The solid state relays also exhibit non-linear voltage-current characteristics.

CN2912059Y discloses a current limiting protection type solid state relay. The main drawback is that this design uses a large number of components for the current limiting circuit with high cost. CN I01465638B discloses a low-power solid state relay, comprising a switch circuit, a first and a second charging circuit, and a discharging circuit. The main drawback of this design is that the switching speed is slower than other devices.

The invention addresses some of the drawbacks of conventional solid state relays, with further related advantages as set forth here.

SUMMARY

An electronic circuit for high speed switching from a power source to a load is disclosed. The circuit includes a switching section, a current limiting section and a voltage limiting section. The switching section comprises a bridge rectifier receiving the load voltage inputs, a MOSFET with a source, drain and gate; the source connected to a negative voltage of the load through the bridge rectifier, the drain connected to the load voltage output. The switching section further includes an isolator circuit comprising an optically-coupled LED. The isolator circuit is configured to receive a logic input signal and generate an isolated output signal based on the logic input signal, and the gate of the MOSFET is driven by the isolated output signal. The current limiting section includes a first set of resistors connecting the voltage drop across the load to the gate of the MOSFET through diodes, wherein the first set of resistors and the diodes are configured to limit current flowing to the gate of the MOSFET. The voltage limiting section comprises a Zener diode connected to the gate of the MOSFET and configured to clamp the voltage to the gate below the maximum voltage rating of the MOSFET.

In some embodiments the current limiting section further comprises a control circuit with a control transistor and a second set of resistors. The control transistor electrically connects the gate of the MOSFET to the source of the MOSFET when the control transistor is ON, thereby turning OFF the MOSFET. The control transistor is turned OFF when the isolator circuit output signal is turned ON.

In one embodiment the control circuit further comprises a first capacitor configured to filter noise from the diodes and a second capacitor configured to maintain the gate of the MOSFET at a positive voltage.

In one embodiment the bridge rectifier is configured to maintain the MOSFET source at a negative voltage and the MOSFET drain at a positive voltage.

In one embodiment the diodes are identical and are configured to allow only positive voltage to the gate of MOSFET with reference to the MOSFET source.

In one embodiment the second set of resistors are low value resistors. In one embodiment one low value resistor is configured to reduce the gate capacitance of the MOSFET and the other low value resistor is configured to limit the current to the Zener diode.

In one embodiment the first set of resistors are high value resistors. In one embodiment a high value pull-up resistor is connected between the diodes and the base of the control transistor.

In various embodiments of the circuit the load is an AC load or a DC load.

In another aspect, a switching circuit includes a bridge rectifier to receive voltage inputs and an optical isolator to receive a logic input signal and generate an output signal based on the logic input signal. The switching circuit also includes a field effect transistor (FET) with a source connected to a negative output of the bridge rectifier, a drain connected to a positive output of the bridge rectifier through a load, and a gate driven by the output signal of the optical isolator. First and second resistors connect the voltage inputs to the gate of the FET through first and second diodes. The first and second resistors and the first and second diodes limit current flowing to the gate of the FET. A Zener diode connected to the gate of the FET limits voltage to the gate of the FET below a maximum voltage rating of the FET.

In another aspect, a method for a switching circuit includes providing a bridge rectifier to receive voltage inputs and providing an optical isolator to receive a logic input signal and generate an output signal based on the logic input signal. A field effect transistor (FET) is provided with a source connected to a negative output of the bridge rectifier, a drain connected to a positive output of the bridge rectifier through a load, and a gate driven by the output signal of the optical isolator. First and second resistors connect the voltage inputs to the gate of the FET through first and second diodes. The first and second resistors and the first and second diodes limit current flowing to the gate of the FET. A Zener diode connected to the gate of the FET limits voltage to the gate of the FET below a maximum voltage rating of the FET.

In another aspect, a method for high speed switching comprises receiving voltage inputs at a bridge rectifier, receiving a logic input signal at an optical isolator and generating an output signal from the optical isolator based on the logic input signal, and driving a gate of a field effect transistor (FET) via the output signal of the optical isolator, wherein a source of the FET is connected to a negative output of the bridge rectifier and a drain of the FET is connected to a positive output of the bridge rectifier through a load. The method further includes limiting current flowing to the gate of the FET through first and second resistors and first and second diodes connecting the voltage inputs to the gate of the FET and limiting voltage to the gate of the FET below a maximum voltage rating of the FET by a Zener diode connected to the gate of the FET.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

FIGS. 1A and 1B show embodiments of the high speed switching solid state relay circuit.

FIG. 2A shows an example where the circuit is used for an AC load configured to switch a pair of motor coils represented by Ll and L3.

FIG. 2B shows schematic waveforms for the circuit of FIG. 2A.

FIG. 3A shows an example configured to run a DC load with various AC or DC input voltages.

FIG. 3B shows schematic waveforms for the circuit of FIG. 3A.

DETAILED DESCRIPTION

While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context dearly dictates otherwise. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on.” Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

In various embodiments, a high speed switching solid state relay circuit is disclosed, that includes a switching section with a metal oxide semiconductor field effect transistor (MOSFET), connecting a power source to an AC or DC load, such as an electric motor, for example. The gate of the MOSFET is driven by a circuit receiving a logic signal. The gate drive circuit further comprises a current limiting section and a voltage limiting section to protect the MOSFET.

In one embodiment, the invention is a high speed switching solid state relay circuit as shown in FIG. 1A. The circuit consists of a switching section, a current limiting section and a voltage limiting section. The switching section includes bridge rectifier BR1 receiving load voltage inputs from a power source and MOSFET Q1 with source S connected to the voltage across the AC_Load through the bridge rectifier BR1, a drain D connected to the load voltage output. In the embodiment of the invention shown in FIG. I A, the circuit is configured with external circuitry to switch a pair of motor coils represented by LI and L3, connected across terminals VI and AC1, and V2 and AC2, respectively, with DC load L2 connected across the bridge rectifier BR1 and the drain D of the MOSFET Ql. MOSFET Q1 includes a Zener diode Z1 connected between the source S and the drain D. The switching circuit further includes an isolator circuit comprising an optically-coupled LED ISO1. The positive side of the DC_Load is connected to the positive output of bridge rectifier BR1 and the negative side of the DC_Load is switched through MOSFET Ql. DC load is represented by coil L2. Zener diode Z1 acts as a flywheel diode to protect the MOSFET Q1 and to dissipate any back emf generated by the windings L1, L2 and L3 when the MOSFET Q1 is OFF. In various embodiments, either one or both the coils L 1 or L3 could be used. In case only one coil is used, the other coil is shorted.

The current limiting section in the circuit includes a first set of resistors R3 and R4 connecting the voltage drop across the load to the gate G of the MOSFET Q1 through diodes Dl. and D2. Resistors R3 and R4 and the diodes Dl and D2 are configured to limit current flowing to the gate G of the MOSFET Ql. The voltage limiting section comprises a Zener diode Z1 connected to the gate G of the MOSFET Ql. Zener diode Z1 is configured to clamp the voltage to the gate G to be below the maximum rated voltage of the MOSFET Q1. In one embodiment the resistors R3 and R4 are high value resistors.

In one embodiment the current limiting section further comprises a control circuit with a control transistor Q2 and a second set of resistors R1 and R2. The control transistor Q2 electrically connects the gate G of MOSFET Q1 to the source S of the MOSFET Ql. In embodiments of the circuits shown in FIG. 1A, the second sets of resistors R1 and R2 are low value resistors. Resistor R1 is a gate resistor configured to constrain the instantaneous current drawn when the MOSFET Q1 is turned ON and R2 is used to limit the current to the Zener diode Z1.

In one embodiment of the invention shown in FIG. 1B, the circuit is configured to take either a DC or AC input to run a DC load L2 connected across the bridge rectifier BR1 and the drain D of the MOSFET Q1. As shown in FIG. 1B, terminals V1 and AC1, and V2 and AC2 are shorted and connected to the AC or DC input.

In some embodiments of the circuits shown in FIGS. 1A and 1B, the control circuit further includes a first capacitor C1 configured to filter noise from the diodes Dl and D2 and a second capacitor C2 configured to maintain the gate G of the MOSFET Q1 at a positive voltage. In embodiments of the circuits shown in FIGS. 1A and 1B, a high value pull-up resistor R5 is connected between the diodes D1 and D2, and the base B of the control transistor Q2 to limit current flowing to the gate G of MOSFET Ql. In one embodiment, the diodes D1 and D2 are identical and are configured to allow only positive voltage to the gate G of the MOSFET Q1 with reference to the source S of the MOSFET Ql. In embodiments of the circuits shown in FIGS. 1A and 1B, the bridge rectifier BR1 maintains the source S of the MOSFET Q1 at a negative voltage and the drain D of MOSFET Q1 at a positive voltage.

In operation of the circuit of one embodiment as shown in FIG. 1A, Coils LI and L3 could be configured to run in alternate polarity of a motor. When MOSFET Q1 is ON, current will flow from the positive output of BRI, through the DC load L2, through Q1 from drain D to source S, then back to the negative output of BRI. The isolator circuit is configured to receive a logic input signal via isolator ISO1, the output of the IS01 is given as input to the base B of the control transistor Q2, and the generated isolated output signal from collector C of the control transistor Q2 is given to the gate G of the MOSFET Q1 via R1. When the logic input is HIGH, MOSFET Q1 is “closed” and allows current to pass through the DC load L2. When the logic input is LOW, MOSFET Q1 is “open”, thereby opening the switch.

In one embodiment gate G of the MOSFET receives inputs via the control transistor Q2, which electrically connects the gate G of MOSFET Q1 to the source S of the MOSFET when the control transistor Q2 is ON, thereby turning OFF the MOSFET Q1 and the load L2 is disconnected. In one embodiment the control transistor Q2 is turned OFF when the isolator circuit ISO1 output signal is turned ON and current passes through MOSFET Q1 and the DC load L2. The control transistor Q2 ensures a “normally open” state when there is no logic input or In_A is held low with reference to In_K.

In various embodiments, the input power to the circuit may be AC or DC power. The load connected to the circuit in various embodiments could also be an AC or a DC load. In the embodiment of the circuit shown in FIG. 1B, the polarity of the input supply is interchangeable across the terminals V1/AC1 and V2/AC2, without affecting operation of the load or damage to the circuit components.

The circuit disclosed with reference to any of the above embodiments may be used in an integrated circuit chip.

Advantages of the invention are that the switching circuit is designed using discrete components: MOSFET, diode bridge, resistors, capacitors, diodes, transistor and an isolator, creating a circuit that functions similar to a solid state relay, at a much lower cost, while providing increased switching speeds and higher power ratings.

While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material the teachings of the invention without departing from its scope as further explained in the following examples, which however, are not to be construed to limit the scope of the invention as delineated by the claims.

Example 1

An example implementation is illustrated in FIG. 2A, which shows how the invention could be configured with external circuitry to switch a pair of AC motor coils represented by L1 and L3. The motor could either use both coils LI and L3, in which case they are configured to run opposite polarity, or it could use only one coil. If only one coil L1 is needed, then AC2 is shorted to V2 and the circuit operates with LI as the only load. FIG. 2B displays voltage waveforms for the circuit of FIG. 2A. In this example, the AC input is 120 Vrms (169 Vpeak) 60 Hz. Coils L1 and L3 are identical in value so their waveforms are the same (Coil≈AC Input/2). The logic input is 5V dc pulsed at 15 Hz. When the logic input is HIGH, Q1 is ON, “closing” the switch and allowing current to pass through the load. When the logic input is LOW, Q1 shuts OFF, “opening” the switch. The control transistor Q2 ensures a “normally open” state when there is no logic input, or In_A is held LOW with reference to In_K.

Example 2

A second implementation of the embodiments of the invention is shown in FIG. 3A, where the figure displays how the circuit could be configured to run a DC load with various input voltages. V1 is connected to AC1 and V2 is connected to AC2. In this configuration, input power can be AC or DC and reversing polarity will have no effect on the operation of the circuit. The positive side of the load is connected to the positive output of BR1 and the negative side of the load is switched through MOSFET Q1. When Q1 is ON, current will flow from the positive output of BR1, through the DC load, through Q1 from drain to source, then back to the negative output of BR1. An advantage of this embodiment is that the circuit operates without damage to the equipment irrespective of whether the input supply is AC or DC. This example simply shows how this invention differs from previous solutions in this one method of using it. FIG. 3B displays voltage waveforms for the circuit of FIG. 3A. As illustrated in Example 1, the logic input is 5 Vdc pulsed at 15 Hz. When the logic input is high, Q1 is ON, “closing” the switch and allowing current to pass through the load. When the logic input is low, Q1 shuts OFF, “opening” the switch. Control transistor Q2 ensures a “normally open” state when there is no logic input, or In_A is held LOW with reference to In_K. 

What is claimed is:
 1. A method for high speed switching, comprising: receiving voltage inputs at a bridge rectifier; receiving a logic input signal at an optical isolator and generating an output signal from the optical isolator based on the logic input signal; driving a gate of a field effect transistor (FET) via the output signal of the optical isolator, wherein a source of the FET is connected to a negative output of the bridge rectifier and a drain of the FET is connected to a positive output of the bridge rectifier through a load, limiting current flowing to the gate of the FET through first and second resistors and first and second diodes connecting the voltage inputs to the gate of the FET; and limiting voltage to the gate of the FET below a maximum voltage rating of the FET by a Zener diode connected to the gate of the FET.
 2. The method of claim 1, further comprising electrically connecting the gate of the FET to the source of the FET through a control transistor and turning the FET off by turning the control transistor on.
 3. The method of claim 2, further comprising turning the control transistor on by turning the optical isolator output signal off.
 4. The method of claim 2, further comprising filtering noise from the first and second diodes using a first capacitor and maintaining the gate of the FET at a positive voltage using a second capacitor.
 5. The method of claim 2, further comprising turning the FET on by turning the control transistor off.
 6. The method of claim 5, further comprising turning the control transistor off by turning the optical isolator output signal on.
 7. The method of claim 5, further comprising constraining, by a third resistor, an instantaneous current drawn when the FET is turned on and limiting current to the Zener diode by a fourth resistor.
 8. The method of claim 7, further comprising using low value resistors for the third and fourth resistors.
 9. The method of claim 1, further comprising using high value resistors for the first and second resistors.
 10. The method of claim 2, further comprising connecting a pull-up resistor between the first and second diodes and a base of the control transistor.
 11. The method of claim 1, further comprising maintaining, by the bridge rectifier, the FET source at a negative voltage and the FET drain at a positive voltage.
 12. The method of claim 1, further comprising configuring the first and second diodes to allow only positive voltage to the FET gate with reference to the FET source.
 13. The method of claim 1, further comprising configuring the load comprising a direct current (DC) load.
 14. The method of claim 1, further comprising configuring the load comprising one or more coils.
 15. The method of claim 1, further comprising configuring the voltage inputs comprising a first voltage input from a power source and a second voltage input from the power source.
 16. The method of claim 1, further comprising configuring an optically-coupled light emitting diode (LED) as the optical isolator.
 17. The method of claim 1, further comprising using the method as a high speed switching solid state relay. 